Method of zone-melting rod-shaped bodies



A. VAN WlERlNGEN 3,085,031

METHOD OF ZONE-MELTING ROD-SHAPED BODIES Filed Feb. 9, 1960 April 9,1963 uuul w- FIG.4

INVENTOR ADRIANUS VAN WIERINGEN United States, Patent F 3,085,031 METHODSF ZfiNE-MELTING ROD=HAPEB BGDiES Adrianne van Wieringen, Nijmegen,Netherlands, assignor to North Americanlhilips (Iompany, Inn, New York,N.Y., a corporation of Delaware Filed-Feh.- 9, 196i), Ser. No. 7,573Claims priority, application Netherlandsl eb. 17, 1959 4. Claims. (Cl.Mil-15).

This invention relates, to methods of zone-melting rodshaped bodies,hereinafter referred to as rods, which methods are to be understood tomean the local heating of such rods so as to obtain a molten zone,whereafter the heat source and the rod are relatively moved so that thezone advances through the rod in the direction of its length. Suchmethods are for instance described in the book Zone Melting by W. G.Pfann (John Wiley and Sons, New York) (1958). I

The material of the rod is melted at the front side of the zone, theso-called melting front, whereas such material recrystallises at therear side of the zone. The melting front is approximately flat and issubstantially at right angles to the direction of length of the rod. Themethod is frequently used for zone refining semi-conductive materials,such as germanium, silicon and intermetallic compounds, but also foradding to such materials certain elements, hereinafter referred to asdopes, which may act upon the conductivity and/ or conductivity type,for instance in a way named zone levelling, and described at pages161-164 of the above mentioned book of Pfann.

Such a dope often consists of a solid material which is disposed on thebody in the form of a pill or powder and dissolved in the molten zone assoon as the latter reaches the location of the pill. It will be evidentthat this method is not applicable if the rod is in a vertical positionsuch as with zone-rnelting without a crucible, sometimes referred to asfloating-zone melting. For this application it has been suggested toprovide the activating material in a cavity provided transversely to thedirection of length of the rod.

In the said methods it may occur that the dope partly evaporates beforebeing absorbed by the melt. It also frequently happens that the pillkeeps floating on the melt for some time so that not only does theevaporation process last longer, but also the moment of the addition ofthe dope to the melt becomes uncertain.

An object of the invention is inter alia to mitigate thesedisadvantages.

According to the invention, the dope is provided in a bore the directionof length of which is at an angle to the melting front, the zone beingled through the rod in a direction such that the melting front reachesthe opening of the bore sooner than the dope. The bore preferably has adepth such that the opening is closed by melting before the dope isreached by the melting front.

The bore, when starting from a lateral face of the rod, may be providedin inclined position with respect to the direction of length, but it isalso possible, when starting at one end of the rod, to provide it in thedirection of length itself.

In order that the invention may be readily carried into effect, twoembodiments will now be described in detail, by way of example, withreference to the accompanying drawing.

FIG. 1 is a diagrammatic sectional view of a device for zone-melting ofa horizontal rod.

FIG. 2 shows a similar device for zone-melting of a vertical rod.

It is to be noted that parts of these devices which are $385,031Fatented- Apr. 9, 1963 ice not essential to the invention, such asenvelopes and supporting members, are omitted.

The device shown in FIG. 1 comprises an elongated crucible 1, in which arod 2, for example. of germanium, is disposed, A movable source ofenergy, represented by a high-frequency coil 3, may bring about in therod a molten zone 4 which may advance in the direction of the arrow.

Let it be assumed that a dope is to be added to the zone at the area.indicated by 5, which dope has been given the form of a pill in theusual manner. Neither the form and the composition of the pill, nor thecomposition of the rod are essential to the invention. As an example therod 2 consists of very pure. zone-refined germanium and has a length of30 cm., a height of 2.5 cm. and a crosssectional area of 4 cm?. Themolten zone 4, having a length of about 3 cm. advances at a rate of 2mm. per minute. The dope may consist of 0.085 mgr. of arsenic and thepill having a diameter of about 1 mm. may consist of germanium in whichthe arsenic has been dissolved.

In inclined direction a bore 6 has been provided, on the bottom of whichthe pill 7 is laid. The bore 6 may have a length of about 1.5 cm. and adiameter of about 2 min. The longitudinal direction of the bore may beat an angle of 45 with the longitudinal direction of the rod 2.

It will be evident that, due to the direction in which the zone 4advances, the melting front 8 reaches the bore at its opening firstly.This opening will thus be closed by melting before the arsenic in thepill 7 has been enabled to evaporate strongly and before the pill 7 willtend to float on the melt.

By a further advance of the melting front 8 of for instance about 1 cm.after the moment at which the opening of the bore 6 was closed, the pill7 is dissolved completely.

The figure shows, in dotted line, a second bore 9 which extends in thedirection of length of the rod. By means of such a bore, which must beat least as deep as the length of the zone 4, it is possible to add adope in a similar manner to that described above.

In the device shown in FIG. 2, a rod 11 is arranged in verticalposition, while an energy source 12 is vertically movable with respectto the rod, resulting in a molten zone 13 which is held between the endsof the rod due to surface tension.

A dope may in this case be added by again providing an inclined bore 14,the opening of which lies higher than its bottom. The dope in the formof a pill 15 is laid in the bore. The molten zone is now advancingdownwards with respect to the rod 11, so that the opening of the boremay be closed by melting by means of the melting front 16 before thepill 15 is strongly heated.

It will be evident that the angle made by the bore with respect to themelting front is important for the effect to be obtained. The largerthis angle, that is to say, the more the bore extends in the directionof length of the rod, the greater is the effect obtained.

What is claimed is:

1. A method of doping an elongated body of semiconductor material,comprising providing from a surface of the body an elongated aperturewhich extends into the body a substantial depth, placing solid dopingmaterial in the aperture within the body through the opening at thesurface, forming in the body a molten zone having front and rearsurfaces with the front surface being inclined toward the said extendedaperture and being closer to the aperture opening at the surface, andadvancing the molten zone through the body in a direction at which itsfront reaches the aperture opening before it reaches and melts the soliddoping material, the angle of inclination 3 between the aperture and thefront surface of the molten zone and the location of the solid dopingmaterial being such that the said front surface reaches and melts thefurthest point of the aperture wall at the surface before it reaches andmelts the solid doping material. I

2. A method as set forth in claim 1 wherein the bore extends into thebody at an acute angle relative to its length direction.

3. A method of doping an elongated body of semiconductor material,comprising providing from a surface of the body an elongated bore whichextends into the body at least partially in the direction of length ofthe body and at an angle to a direction transverse to the body, placingsolid doping material at the bottom of the bore within the body throughthe opening at the surface, forming in the body a transverse molten zonehaving front and rear surfaces with the front surface closer to the boreopening than its bottom, and advancing the molten zone through the bodyin its length direction at which the molten front reaches the boreopening before the front reaches and melts the solid doping material,the angle of the bore and its depth being such that the said moltenfront reaches and melts the furthest point of the bore wall at thesurface before it reaches and melts the solid doping material.

4. A method as set forth in claim 3 wherein the bore extends into thebody from one end in its length direction.

References Cited in the file of this patent UNITED STATES PATENTS2,739,088 Pfann Mar. 20, 1956 2,789,039 Jensen Apr. 16, 1957 2,813,048Pfann Nov. 12, 1957 2,841,559 Rosi July 1, 1958 2,898,249 Jensen Aug. 4,1959 2,902,350 Jenny et a1 Sept. 1, 1959 OTHER REFERENCES Kolb et al.:Electrochemical Society Journal, vol. 106,

20 No. 7, July 1959, pages 597-599.

1. A METHOD OF DOPING AN ELONGATED BODY OF SEMICONDUCTOR MATERIAL,COMPRISING PROVIDING FROM A SURFACE OF THE BODY AN ELONGATED APERTUREWHICH EXTENDS INTO THE BODY A SUBSTANTIAL DEPTH, PLACING SOLID DOPINGMATERIAL IN THE APERTURE WITHIN THE BODY THROUGH THE OPENING AT THESURFACE, FORMING IN THE BODY A MOLTEN ZONE HAVING FRONT AND REARSURFACES WITH THE FRONT SURFACE BEING IN CLINED TOWARD THE SAID EXTENDEDAPERTURE AND BEING CLOSER TO THE APERTURE OPENING AT THE SURFACE, ANDADVANCING THE MOLTEN ZONE THROUGH THE BODY IN A DIRECTION AT WHICH ITSFRONT REACHES THE APERTURE OPENING BEFORE IT REACHES AND MELTS THE SOLIDDOPING MATERIAL, THE ANGLE OF INCLINATION BETWEEN THE APERTURE AND THEFRONT SURFACE OF THE MOLTEN ZONE AND THE LOCATION OF THE SOLID DOPINGMATERIAL BEING SUCH THAT THE SAID FRONT SURFACE REACHES AND MELTS THEFURTHEST POINT OF THE APERTURE WALL AT THE SURFACE BEFORE IT REACHES ANDMELTS THE SOLID DOPING MATERIAL.